Traditionally, propylene glycol (PG and ethylene glycol (EG) have been produced from petrochemical sources. The current industrial or commercial route to produce propylene glycol is by the hydration of propylene oxide converted from petroleum-derived propylene by either the chlorohydrin process or the hydroperoxide process (A. E., Martin, F. H. Murphy, 4th ed. Kirk-Othmer Encyclopedia of Chemical Technology, vol. 17, Wiley, New York, 1994. p. 715; D. T. Trent, 4th ed. Kirk-Othmer Encyclopedia of Chemical Technology, vol. 20, Wiley, New York, 1996, p. 271). The commercial production of ethylene glycol involves the hydration of ethylene oxide, made by the oxidation of ethylene. Propylene and ethylene are industrial by-products of gasoline manufacture, for example as by-products of fluid cracking of gas oils or steam cracking of hydrocarbons.
The world's supply of petroleum is being depleted at an Increasing rate. Eventually, demand for petrochemical derived products will outstrip the supply of available petroleum. When this occurs, the market price of petroleum and, consequently, petroleum derived products will likely increase, making products derived from petroleum more expensive and less desirable. As the available supply of petroleum decreases, alternative sources and, in particular, renewable sources of comparable products will necessarily have to be developed. One potential renewable source of feedstocks for producing such comparable products is bio-based matter, such as agricultural and forestry products. Use of bio-based products may potentially counteract, at least in part, the problems associated with depletion of the petroleum supply.
Catalytic hydrogenolysis (hydrocracking) conversion of carbohydrate-based feedstocks, such as five and six carbon-unit polysaccharides and/or sugar alcohols (conventionally, glycerol, glycols, or sorbitol), involves reacting the carbohydrate-based feedstocks with hydrogen to produce compounds that are referred to as “polyols” or “polyhydric alcohols.” The reaction with hydrogen breaks down the carbohydrate molecules into fragments of lower molecular weight.
For instance, U.S. Pat. No. 5,206,927 describes a homogeneous process for hydrocracking carbohydrates in the presence of a soluble, transition metal catalyst to produce lower polyhydric alcohols. A carbohydrate is contacted with hydrogen in the presence of a soluble transition metal catalyst and a strong base at a temperature of from about 25° C. to about 200° C. and a pressure of from about 15 to about 3000 psi. Other processes, for example, in U.S. Pat. Nos. 5,276,181 and 5,214,219, involve hydrogenolysis of glycerol using a copper and zinc catalyst in addition to a sulfided ruthenium catalyst at a pressure over 2100 psi and temperature between 240-270° C. U.S. Pat. No. 5,616,817 describes a process of preparing 1,2 propanediol (propylene glycol) by catalytic hydrogenolysis of glycerol at elevated temperature and pressure using a catalyst comprising the metals cobalt, copper, manganese and molybdenum. German patent DE 541362 describes the hydrogenolysis of glycerol with a Nickel catalyst, while U.S. Pat. No. 4,476,331 describes a two stage method of hydrocracking carbohydrates (for example glucose), wherein a modified ruthenium catalyst is used for hydrocracking sorbitol to produce glycerol derivatives. European Patent applications EP-A-0523 014 and EP-A-0 415 202 describe a process for preparing lower polyhydric alcohols by catalytic hydrocracking of aqueous sucrose solutions at elevated temperature and pressure using a catalyst whose active material comprises the metals cobalt, copper and manganese. Persoa & Tundo (Ind. Eng. Chem. Res. 2005, 8535-8537) describe a process for converting glycerol to 1,2-propanediol by heating under low hydrogen pressure in presence of Raney nickel and a liquid phosphonium salt. Selectivities toward 1,2-Propanediol as high as 93% were reported, but required using a pure glycerol and long reaction times (20 hrs.), Crabtree et al. (Hydrocarbon processing, February 2006, pp. 87-92) describe a phosphine/precious metal salt catalyst that permit a homogenous catalyst system for converting glycerol into 1,2-PD. However, low selectivity (20-30%) was reported. Other reports indicate use of Raney Copper (Montassier et al. Bull. Soc. Chim. Fr. 2 1989 148; Stud. Surf. Sci. Catal. 41 1988 165), copper on carbon (Montassier et al. J. Appl. Catal. A 121 1995 231)), copper-platinum and copper ruthenium (Montassier et al. J. Mol. Catal. 70 1991 65). Other homogenous catalyst systems such as tungsten and Group VIII metal-containing catalyst compositions have been also tried (U.S. Pat. No. 4,642,394). Miyazawa et al. (J. Catal. 240 2006 213-221) & Kusunoki et al. (Catal. Comm. 6 2005 645-649) describe a Ru/C and ion exchange resin for conversion of glycerol in aqueous solution. Again their process however, results in low conversions of glycerol (0.9-12.9%). Still other processes are described, for example, in U.S. Pat. Nos. 7,928,148; 6,479,713; 6,291,725, or 5,354,914, the contents of each are incorporated herein by reference in their entirety.
Some processes of hydrocracking complex mixtures of higher carbohydrates involve reacting reagents under alkaline conditions. According to some processes, the pH value of a resulting polyol product mixture, containing propylene glycol and ethylene glycol, is neutralized with a strong acid, such as H2SO4 or HCl, after the reaction is completed. This unfortunately can contribute to problems in subsequent purification. By introducing a strong acid (e.g., pH≦1.5 or 2.0), one protonates the salts of organic acids in the mixture.
Polyols produced by hydrogenolysis of bio-derived feedstock often comprise a mixture of several polyols having a lower average molecular weight than the starting material. One of the recognized problems in the conversion of polyols, such as sugars and glycerol to polyhydric alcohols, such as propylene glycol and ethylene glycol by hydrogenous or by hydrocracking results in formation of not only these alcohols, but also several other diol compounds, which reduces the purity of the desired component. These unwanted products are recovered along with propylene glycol and ethylene glycol, and include for example: 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol and 2,4-pentanediol. Such impurities of the polyol product, mixture (derivatives) present a problem for sale and use of the product.
Due to the similarity in boiling points, these diols are very difficult to separate from propylene glycol by distillation. Hence, the separation of substantially pure propylene glycol or ethylene glycol from these other polyhydric alcohols by ordinary rectification is difficult. For example, the butane diols (BDO), pentane diols (PDO) of various isomeric forms (e.g., 2, 3 BDO: 1, 3 PDO) are the most difficult to separate from propylene glycol using current distillation processes because their boiling point temperatures are very close to that of propylene glycol (i.e., 185° C.-189° C. The boiling points of many of these components are shown in Table A.
TABLE APolyols produced by Hydrocracking of SorbitolPolyolWeight Percent (%)Boiling Point (° C.).2,3-Butanediol3.5182Propylene glycol16.5 1871,2-Butanediol2.0192Ethylene glycol25.2 1981,3-Butanediol2.72062,3-Hexanediol—2061,2-Pentanediol—2101,4-Pentanediol—2201,4-Butanediol2.12301,5-Pentanediol0.1242Diethylene glycol2.22451,6-Hexanediol—250Triethylene glycol2.1285Glycerol38.8 2901,2,4-Butanetriol4.8190/18 mm
The differences in volatility of propylene glycol compared to 2,3-butanediol or 1,2 butanediol are very small. The relative volatility is so low that a large number of theoretical plates are required to produce high purity polyols. As shown in Tables B and C, the number of plates required to achieve 99% purity is very large, requiring the use of very tall distillation columns (55 trays for 2,3-Butanediol and 88 trays for 1,2-Butanediol) and high energy inputs.
TABLE BTheoretical and Actual Plates Required vs. Relative volatilityfor Separation of Propylene Glycol and 2,3-Butanediol.Relative VolatilityTheoretical PlatesActual Plates, 75% Efficiency1.2541551.3531421.4525341.5023311.701824
TABLE CTheoretical and Actual Plates Required vs. Relative volatilityfor Separation of Propylene Glycol and 1,2-Butanediol.Relative VolatilityTheoretical PlatesActual Plates, 75% Efficiency1.1566881.523312.014193.09123.5811
Some approaches for separating and purifying a hydrogenolysis reaction mixture are discussed, for example, in commonly assigned U.S. Pat. No. 8,143,458, to Kalagias et al., and U.S. Patent Publication No. 2009/0120878A1 to Hilaly et al. U.S. Pat. No. 8,143,458 describes a process for separating ethylene glycol or propylene glycol from mixtures containing the ethylene glycol or the propylene glycol and other polyols using polar compounds by means of an addition of a polar solvent and extractive distillation. U.S. Patent Publication 2009/0120878A1 describes methods of separating butanediol compounds, particularly 1,2-butanediol and 2,3-butanediol from a mixture of polyhydric alcohols using a simulated moving bed chromatography as a means to achieve a purified, commercial grade bio-based propylene glycol. The contents of each of the foregoing patent documents are herein incorporated.
The prior art describes the difficulty of refining and purifying propylene glycol or ethylene glycol from a hydrogenolysis product mixture. A compounding difficulty however arises from the fact that in distilling the entire polyol product mixture to remove the impurities of other undesired polyhydric alcohols, additional reactions occur that give rise to aldehydes, ketones, esters and epoxides. Polyol products that can contain these compounds are commercially unacceptable in terms of the purity and quality of propylene glycol yielded. For example, in distilling out, epoxides such as propylene oxide and glycidol can be formed. These two epoxides in particular are of concern for certain established uses and commercially important applications of propylene glycol, at least, for the reason that these substances are listed under the State of California's “The Safe Drinking Water and Toxic Enforcement Act of 1986”—more commonly known as Proposition 65—as being known to California to cause cancer. Consequently, having a biobased, drop-in replacement propylene glycol for a petroleum-based or -derived propylene glycol will depend, for certain markets and end uses at least, on developing an economical process of separating polyethylene glycol and/or ethylene glycol from other polyhydric alcohols that also satisfactorily addresses this problem.
International Application Serial No. PCT/US2012/026728, the contents of which are incorporated herein by reference, proposes several methods for solving this further problem. For instance, the application describes a process for distilling a product mixture comprised of biobased propylene glycol, biobased ethylene glycol or a combination thereof and which further includes one or both of propylene oxide and glycidol, so that a distilled biobased glycol product stream is produced which is substantially free of both propylene oxide and glycidol. Epoxide removal is thus integrated into the refining process for a crude reaction product, to produce the desired biobased, commercially acceptable glycol product.